Cobalt oxide is a promising earth-abundant electrocatalyst for water splitting; however, the structural complexity of oxides coupled with the difficulty of characterizing it in its operating environment means that fundamental understanding of its catalytic properties remains poor. In this study, we go beyond vacuum studies and investigate the morphological evolution of a CoO _x /Au(111) model system from intermediate to high pressures of H ₂ O vapor by means of scanning tunneling microscopy and near-ambient pressure and vacuum X-ray photoelectron spectroscopy. At elevated H ₂ O pressure, we describe the formation of a well-defined Co(OH) ₂ nanoisland morphology with cobalt in the +2 oxidation state. In contrast, the presence of O ₂ , in air and liquid water, results in only partially hydroxylated Co ³⁺ phases comprising sheets of the CoO(OH _x ) trilayer, corresponding to a single sheet of cobalt(III)oxyhydroxide. We conclude that the oxyhydroxide structure, known to be the catalytically active phase for the oxygen evolution reaction, is stabilized by aerobic conditions, which inhibits further transformation into the catalytically inactive cobalt(II)hydroxide.